Video coding method for a plasma display panel

ABSTRACT

The invention proposes a novel technique for coding video for a plasma display panel, which aims to improve the use of rotating code. The method forming the subject of the invention carries out a coding of the highest grey level V 1  over the entirety of the subscans while favouring the subscans whose illumination time is the lowest. The coding of the lowest grey level V 2  is achieved using the common value COMMAX which results from the coding of the value V 1  if the specific value SPEMAX is greater than the difference D of the grey levels. If SPEMAX is greater than the difference D, then the coding of the common value COMMIN corresponds to the lowest value V 2.

This application claims the benefit under 35 U.S.C. §365 ofInternational Application PCT/FR00/02498, filed Sep. 11, 2000, whichclaims the benefit of French Patent Application No. 99/11900, filed Sep.23, 1999.

The invention relates to a method of coding video for a plasma displaypanel. More particularly, the invention relates to the coding of thegrey levels of a type of panel with separate addressing and sustaining.

Plasma display panels, called hereafter PDPs, are flat-type displayscreens. There are two large families of PDPs, namely PDPs whoseoperation is of the DC type and those whose operation is of the AC type.In general, PDPs comprise two insulating tiles (or substrates), eachcarrying one or more arrays of electrodes and defining between them aspace filled with gas. The tiles are joined together so as to defineintersections between the electrodes of the said arrays. Each electrodeintersection defines an elementary cell to which a gas spacecorresponds, which gas space is partially bounded by barriers and inwhich an electrical discharge occurs when the cell is activated. Theelectrical discharge causes an emission of UV rays in the elementarycell. Phosphors (red, green or blue) deposited on the walls of the cellconvert the UV rays into visible light.

In the case of AC-type PDPs, there are two types of cell architecture,one called a matrix architecture and the other called a coplanararchitecture. Although these structures are different, the operation ofan elementary cell is substantially the same. Each cell may be in theignited or “on” state or in the extinguished or “off” state. A cell maybe maintained in one of these states by sending a succession of pulses,called sustain pulses, throughout the duration over which it is desiredto maintain this state. A cell is turned on, or addressed, by sending alarger pulse, usually called an address pulse. A cell is turned off, orerased, by nullifying the charges within the cell using a dampeddischarge. To obtain various grey levels, use is made of the eye'sintegration phenomenon by modulating the durations of the on and offstates using subscans, or subframes, over the duration of display of animage.

In order to be able to achieve temporal ignition modulation of eachelementary cell, two so-called “addressing modes” are mainly used. Afirst addressing mode, called “addressing while displaying”, consists inaddressing each row of cells while sustaining the other rows of cells,the addressing taking place row by row in a shifted manner. A secondaddressing mode, called “addressing and display separation”, consists inaddressing, sustaining and erasing all of the cells of the panel duringthree separate periods. For more details concerning these two addressingmodes, a person skilled in the art may, for example, refer to U.S. Pat.Nos. 5,420,602 and 5,446,344.

FIG. 1 shows the basic time division of the “addressing and displayseparation” mode for displaying an image. The total display time Ttot ofthe image is 16.6 or 20 ms, depending on the country. During the displaytime, eight subscans SC1 to SC8 are effected so as to allow 256 greylevels per cell, each subscan making it possible for an elementary cellto be “on” or “off” for an illumination time Tec which is a multiple ofa value To. Hereafter, reference will be made to an illumination weightp, where p corresponds to an integer such that Tec=p*To. The totalduration of a subscan comprises an erasure time Tef, an address time Taand the illumination time Tec specific to each subscan. The address timeTa can also be decomposed into n times an elementary time Tae, whichcorresponds to the addressing of one row. Since the sum of theillumination times Tec needed for a maximum grey level is equal to themaximum illumination time Tmax, we have the following equation:Ttot=m*(Tef+n*Tae)+Tmax, in which m represents the number of subscans.FIG. 1 corresponds to a binary decomposition of the illumination time.This binary representation has numerous drawbacks. A problem of falsecontours (or “contouring”) has been identified for quite some time.

The problem of false contours stems from the proximity of two areaswhose grey levels are very close but whose illumination instants aredecorrelated. The worst case corresponds to a transition between thelevels 127 and 128. This is because the grey level 127 corresponds to anillumination for the first seven subscans SC1 to SC7, while the level128 corresponds to the illumination of the eighth subscan SC8. Two areasof the screen placed one beside the other, having the levels 127 and128, are never illuminated at the same time. When the image is staticand the observer's eyes do not move over the screen, temporalintegration takes place relatively well (if any flicker effect isignored) and two areas with relatively close grey levels are seen. Onthe other hand, when the two areas move over the screen (and/or theobserver's eye moves), the integration time slot changes screen area andis shifted from one area to another for a certain number of cells. Theshift in the eye's integration time slot from an area of level 127 to anarea of level 128 has the effect of integrating so that the cells areoff over the period of one frame, which results in the appearance of adark contour of the area. Conversely, shifting the eye's integrationtime slot from an area of level 128 to an area of level 127 has theeffect of integrating so that the cells are lit over the duration of oneframe, which results in the appearance of a light contour of the area.This phenomenon is manifested, when working on pixels consisting ofthree elementary cells (red, green and blue), as false colouredcontours.

The explained phenomenon occurs at all level transitions where theswitched illumination weights are totally or almost totally different.Switchings of high weight are more annoying than switchings of lowweight because of their magnitude. The resulting effect may beperceptible to a greater or lesser extent depending on the switchedweights and on their positions. Thus, the contouring effect may alsooccur with levels that are quite far apart (for example 63-128), but itis less shocking for the eye as it then corresponds to a very visiblelevel (or colour) transition.

To remedy this problem of contouring, several solutions have beenimplemented. One solution consists in “breaking up” the high weights,this involving adding extra subscans. Only the total time of display ofthe image Ttot=m*(Tef+n*Tae)+Tmax remains fixed, thereby resulting in adrop in the time Tmax (since Tef and Tae are incompressible durations)and hence a drop in maximum brightness of the screen. It is possible touse up to 10 subscans while having correct brightness. With 10 subscans,the maximum illumination time Tmax is, currently, 30% of the total timewhereas the erasure and address time is of the order of 70%. FIG. 2represents an example of addressing using 10 subscans SC1 to SC10 inwhich the high weights are broken up into two.

In order to reduce the considerable transitions and to increase thenumber of subscans without reducing the brightness of the screen, onetechnique consists in simultaneously scanning two successive rows forcertain illumination values. We then have the following equationTtot=m₁*(Tef+n*Tae)+m₂*(Tef+n/2*Tae)+Tmax. The erasure time Tef beingnegligible relative to n*Tae, we have the equivalenceTtot≅(m₁+m₂/2)*(Tef+n*Tae)+Tmax. These simultaneous subscans halve theaddress time, and thus make it possible to add extra subscans withoutreducing Tmax. FIG. 3 represents an example of addressing with 11subscans S1 to S11. Subscans S1 and S2 corresponding to the lowestillumination times are carried out on two rows at the same time so as toobtain an overall address time for these two subscans which is equal tothe address time of a single subscan. If subscans common to twosuccessive rows are performed for the illumination weights 1, 2, 4 and8, it is possible to obtain 12 subscans so as to eliminate thetransitions of weight 64. The problem with this solution is however theloss of resolution due to the simultaneous scanning of two rows.

With regard to the principle of subscans scanning two rows at the sametime, one solution consists in using a rotating-code coding, ormultiple-representation coding. FIG. 4 illustrates a rotating-codecoding using twelve subscans S1 to S12 with which are associated thefollowing illumination weights: 1, 2, 4, 6, 10, 14, 18, 24, 32, 40, 48and 56. One effect of the rotating code is to soften the high-weightswitchings by reducing the number of weights switched when switching ahigh weight. To obtain the twelve subscans, a simultaneous scan of tworows is performed for the weights 2, 6, 14 and 24. Such a codefurthermore allows a multiple representation of the numbers:34=32+2=24+10=24+6+4=18+14 +2= . . . etc. This multiple representationof the numbers makes it possible to code the grey levels present on thetwo rows scanned at the same time in such a way that the weights 2, 6,14 and 24 are identical. The person skilled in the art may refer toEuropean Patent Application No. 0 874 349 for further details regardingthis technique. Although the effect of softening the switching of a highweight is attenuated by the multiple coding, problems of loss ofresolution and of contouring still arise when it is not possible to havesuitable coding between two simultaneously addressed cells.

In European Patent Application EP-A-0 945 846, it is proposed, amongother things, that the use of rotating codes be optimized so as to beable to increase the number of subscans considerably. The technique usedconsists in decomposing the grey levels GL1 and GL2, for two cellsplaced on two adjacent rows, into a common value CV and into a specificvalue SV1 and SV2. Thus, we have:

GL 1=SV 1+CV,

GL 2=SV 2+CV,

with, when GL1<GL2:

SV 1=α*GL 1,

SV 2=D+α*GL 1,

CV=½(GL 1+GL 2−SV 1−SV 2),

and where α is a coefficient to be defined as a function of the type ofcode used,

D is equal to the difference GL2−GL1 after rounding, for examplerounding to 5.

In most cases, the rounding to 5 of the difference makes it possible tolimit the error to plus or minus 1 bit. However, such a system islimited by a deviation in the maximum difference value. It is thennecessary to have for example SV1=0, SV2=maximum value, and CV chosen soas to minimize the error. The probability of frequency of occurrence ofsuch a case depends essentially on the choice of the number of subscansspecific to each cell and of the duration of illumination of the saidsubscans.

A second limitation of such a method of coding stems from the dispersionof the various codings for one and the same value. Specifically, thecoding variations no longer depend on each cell but on each pair ofcells independently of the neighbouring pair. The phenomenon ofcontouring is strongly attenuated inside the pair of cells but theattenuation of the false contour is less with the neighbouring pairs.The person skilled in the art will note, on reading EP-A-0 945 846, thatto minimize this limitation, it is advisable to use a common part whichis the largest possible, this having the effect of increasing theprobability of error due to the limitation stemming from the deviationin the maximum value.

The invention proposes a novel scanning technique aimed at improving theuse of rotating code. The grey level coding method forming the subjectof the invention carries out a coding which favours one choice fromamong two possible codes as a function of the grey levels associatedwith each cell. The two codes use equivalent criteria so that thedisparity between the two codes is minimized. According to theinvention, the coding of the highest grey level over the entirety of thesubscans has priority. If the coding over the entirety of the subscansis not advisable, the lowest grey level is coded over the subscanscommon to the two cells of the pair. In both cases, the subscanscorresponding to the low illumination weight are favoured. Such a methodcarries out various codings for one and the same value while retaininggreat proximity between the various codes.

The subject of the invention is a method of displaying a video image ona plasma display panel for a duration of display, the said panelcomprising a plurality of cells arranged in rows and columns, each cellbeing lit for a duration lying between zero and a maximum display timecorresponding to the maximum brightness of a cell for a given brightnesssetting, the total time of illumination of a cell being divided intoseveral illumination periods corresponding to various subscans amongwhich are distinguished first subscans specific to an addressing of eachcell and second subscans common to two cells arranged on neighbouringrows, such that, for a pair of cells sharing the same second subscans,the grey levels GL1 and GL2 of the said cells are decomposed into acommon value CV and into a specific value SV1 and SV2 with the aid ofthe following equation: GL1=CV+SV1 and GL2=CV+SV2, with GL1 greater thanor equal to GL2, the said method comprising the following steps:

E1: coding of the highest grey level GL1 over the entirety of thesubscans while favouring the subscans whose illumination time is thesmallest;

E2: extraction of the specific value VS1 corresponding to the coding ofstep E1;

E3: coding of the lowest grey level by using the common value CVresulting from step E1 if the specific value SVl extracted in step E2 isgreater than the difference GL1−GL2.

In certain cases, the following step is carried out:

E4: coding of the common value CV as being equal to the lowest greylevel GL2 if the specific value SV1 extracted in step E2 is less thanthe difference GL1−GL2, then calculation of a new value SV1=GL1−GL2.

Preferably, if the maximum value encodable with the aid of the firstsubscans is less than the difference GL1−GL2, then the common value CVis equal to the lowest grey level GL2, and the specific value SV1 isequal to the maximum value encodable on the first subscans.

To control the error due to the use of common subscans, prior to anycoding operation, the value 1 may possibly be added to and/or subtractedfrom one or both grey levels GL1 et GL2 so that the difference GL1−GL2is a multiple of five.

According to a particular embodiment, the display durations associatedwith the first subscans correspond to the product of an elementaryduration times respectively the factors: 5, 10, 20, 30, 40, 45, and thedisplay durations associated with the second subscans correspond to theproduct of the elementary duration times respectively the factors: 1, 2,4, 7, 13, 17, 25, 36.

The invention also relates to a plasma display panel comprising aplurality of cells arranged in rows and columns, each cell being lit fora duration lying between zero and a maximum display time correspondingto the maximum brightness of a cell for a given brightness setting, thetotal time of illumination of a cell being divided into severalillumination periods corresponding to various subscans among which aredistinguished first subscans specific to an addressing of each cell andsecond subscans common to two cells arranged on neighbouring rows, suchthat, for a pair of cells sharing the same second subscans, the greylevels GL1 and GL2 of the said cells are decomposed into a common valueCV and into a specific value SV1 and SV2 with the aid of the followingequation: GL1=CV+SV1 and GL2=CV+SV2, with GL1 greater than or equal toGL2, the said panel comprising a grey level encoding device comprising:

a first coding circuit for coding the highest grey level GL1 over theentirety of the subscans while favouring the subscans whose illuminationtime is the lowest;

a means for extracting a common value CV and a specific value SV1exiting the first coding circuit;

a selection and calculation circuit for carrying out the coding of thelowest grey level by using the common value CV exiting the first codingcircuit if the specific value SV1 extracted from the first codingcircuit is greater than the difference GL1−GL2.

The other features of the method are also transposed over to the device.

The invention will be better understood and other features andadvantages will become apparent from reading the description whichfollows, the description making reference to the appended drawings amongwhich:

FIGS. 1 to 4 represent temporal distributions of subscans during thedisplaying of an image according to the state of the art,

FIG. 5 represents the temporal distribution of subscans according to apreferred embodiment,

FIG. 6 represents a grey level coding algorithm according to theinvention,

FIG. 7 represents a processing circuit implementing the coding algorithmaccording to the invention,

FIGS. 8 to 10 represent details of the circuit of FIG. 7,

FIG. 11 represents a plasma display screen implementing the invention.

For representational reasons, the temporal distribution of the subscansuses significant proportions which do not correspond to an exact linearscale.

FIG. 5 shows a preferred temporal distribution, for which an embodimentwill be described. This temporal distribution comprises first subscansFSC specific to each row, which make it possible to address each cell ofthe screen individually. In the preferred example, six first subscansFSC are used, with which the respective illumination weights 5, 10, 20,30, 40 and 45 are associated. Such a choice makes it possible to have amaximum difference value of 150 over 256 grey levels. A statisticalstudy on video images makes it possible to determine that theprobability of error due to the maximum difference value is much lessthan 5%.

Second subscans SSC simultaneously address to adjacent rows. In thepreferred example, there are eight second subscans SSC with which areassociated the respective weights 1, 2, 4, 7, 13, 17, 25 and 36.

The method of coding the grey levels for each pair of cells will now bedescribed with the aid of the algorithm of FIG. 6. The algorithm beginswith two known grey levels GL1 and GL2 associated respectively with afirst and a second cell.

In a first step 101, the absolute value of the difference between GL1and GL2 is calculated. This difference |GL1−GL2| is then rounded to fiveto minimize the error, the rounded difference being called D hereinbelow.

In a second step 102, the values V1 and V2 corresponding respectively tothe levels GL1 and GL2 are calculated. These values V1 and V2 aredetermined on the one hand as a function of the rounding carried out onthe difference |GL1−GL2| and on the other hand as a function of theminimum and maximum values of GL1 and GL2. In the example described, therounding of the difference and the modifying of V1 and V2 is performedaccording to the following table:

Last digit of |GL1-GL2| D V1 V2 0 0 Max(GL1, GL2) Min(GL1, GL2) 1 0Max(GL1, GL2) − 1 Min(GL1, GL2) 2 0 Max(GL1, GL2) − 1 Min(GL1, GL2) + 13 5 Max(GL1, GL2) + 1 Min(GL1, GL2) − 1 4 5 Max(GL1, GL2) Min(GL1, GL2)− 1 5 5 Max(GL1, GL2) Min(GL1, GL2) 6 5 Max(GL1, GL2) − 1 Min(GL1, GL2)7 5 Max(GL1, GL2) − 1 Min(GL1, GL2) + 1 8 0 (diz. sup.) Max(GL1, GL2) +1 Min(GL1, GL2) − 1 9 0 (diz. sup.) Max(GL1, GL2) Min(GL1, GL2) − 1

After calculation of the values V1 and V2 comes a third step 103 ofencoding. The encoding carried out consists in performing on the onehand the coding of the value V1 over the entirety of the subscans FSCand SSC while favouring the subscans corresponding to the lowestillumination weights and on the other hand by performing the coding ofthe value V2 on the second subscans SSC while favouring the subscanscorresponding to the low illumination weights. After encoding, a six-bitword SPEMAX is available, corresponding to the first subscans FSC usedto code the value V1. The value corresponding to the sum of the weightsof the first subscans activated in SPEMAX is associated with the wordSPEMAX. An eight-bit word COMMAX is also available, corresponding to thesecond subscans SSC used to code the value VI. The value correspondingto the sum of the weights of the second subscans activated in COMMAX isassociated with the word COMMAX. Finally, an eight-bit word COMMIN isavailable, corresponding to the second subscans SSC used to code thevalue V2. The value corresponding to the sum of the weights of thesecond subscans activated in COMMIN is associated with the word COMMIN.

On completion of the third step 103, a first test 104 is performed. Thefirst test 104 checks that the absolute value of the rounded differenceD is less than the maximum difference value DMAX, in the present exampleDMAX=150. The person skilled in the art will understand that this testcan be carried out as soon as the first step 101 has been carried outand that it is not necessary to wait for the end of the third step 103in order to be able to carry it out. If the first test 104 indicatesD>DMAX, then the fourth step 105 is performed, otherwise, a second test106 is performed.

The second test 106 checks whether or not the value which corresponds toSPEMAX is less than the difference D. If the value of SPEMAX is lessthan the difference D then a fifth step 107 is performed, otherwise asixth step 108 is performed.

The fourth to sixth steps 105, 107 and 108 are assignment steps whichdetermine three words Si, Sj and COM. The word Si is a six-bit wordwhich corresponds to the coding of the first subscans FSC for the cellhaving the highest grey level. The word Sj is a six-bit word whichcorresponds to the coding of the first subscans FSC for the cell havingthe lowest grey level. The word COM is an eight-bit word whichcorresponds to the coding of the second subscans SSC which are common toboth cells.

The fourth step 105 assigns the word Si so that it corresponds to thecarrying out of all the first subscans FSC, the word Sj so that itcorresponds to the carrying out of none of the first subscans FSC, andthe word COM so that it is identical to the word COMMIN. This fourthstep 104 amounts to encoding the values V1 and V2 by considering thatV1=V2+DMAX. The previously mentioned case of error due to the maximumdifference now holds. The combination of a maximum difference value(DMAX=150 in our example) with the coding used creates an error only inrespect of the largest value which corresponds to a high luminance andhence to lesser perception for the human eye.

The fifth step 107 assigns the word Si so that it corresponds to thecarrying out of the first subscans FSC whose total illumination weightcorresponds to the difference D. The fifth step 107 also assigns theword Sj so that it corresponds to the carrying out of none of the firstsubscans FSC, and the word COM so that it is identical to the wordCOMMIN.

The sixth step 108 assigns the word Si so that it is identical to theword SPEMMAX, and the word COM so that it is identical to the wordCOMMAX. The word Sj is defined so as to correspond to an illuminationcorresponding to the value of the word SPEMAX minus the difference D.

On completion of the fourth to sixth steps 105, 107 and 108, a thirdtest 109 is performed to determine which grey level GL1 or GL2 is thehighest so as in seventh and eighth steps 110 and 111 to match the wordsSi and Sj to the words S1 and S2 which correspond to the first subscansFSC for the respective levels GL1 and GL2.

The algorithm thus described is repeated for each pair of cells whosesecond subscans FSC are common.

In order to better understand the manner of operation of the methodforming the subject of the invention, a few exemplary applications willnow be described. For greater clarity, the various words correspondingto the coding of the subscans are represented in the form of a sum ofvalues, each value corresponding to the activation of the subscanassociated with the said value.

FIRST EXAMPLE

GL1=130, GL2=124

|GL1−GL2|=6=>D=5

V1=130=1+2+5+7+10+13+17+20+25+

COMMAX=1+2+7+13+17+25

SPEMMAX=5+10+20+30

Value(SPEMMAX)=65

In this first example, the difference D is less than DMAX and D is lessthen SPEMAX, it is the sixth step 108 which is carried out, we willtherefore have:

Si=5+10+20+30

Sj=10+20+30

COM=1+2+7+13+17+25

GL1 being greater than GL2, we obtain:

Code(GL1)=1+2+5+7+10+13+17+20+25+30

Code(GL2)=1+2+7+10+13+17+20+25+30

SECOND EXAMPLE

GL1=62, GL2=130

|GL1−GL2|=68=>D=70

V1=131=4+5+7+10+13+17+20+25+30

SPEMMAX=5+10+20+30

Value(SPEMMAX)=65

V2=61

COMMIN=2+4+13+17+25

In this second example, the difference D is less than DMAX and D isgreater than SPEMAX, it is the fifth step 107 which is carried out, wewill therefore have:

Si=10+20+40

Sj=0

COM=2+4+13+17+25

GL1 being less than GL2, we obtain:

Code(GL1)=2+4+13+17+25

Code(GL2)=2+4+10+13+17+20+25+40

THIRD EXAMPLE

GL1=53, GL2=242

|GL1−GL2|=189=>D=190

V=242, V2=52

COMMIN=1+4+7+13+17

In this third example, the difference D is greater than DMAX, it is thefourth step 105 which applies.

Si=5+10+20+30+40+45

Sj=0

COM=1+4+7+13+17

GL1 being less than GL2, we obtain:

Code(GL1)=1+4+7+13+17

Code(GL2)=1+4+5+7+10+13+17+20+30+40+45

In these various examples, the person skilled in the art can see thatthe various pairs of grey levels GL1 and GL2 share a large number ofcommon subscans, in particular when the said levels GL1 and GL2 are veryclose. To this prime effect is added a grouping of the coding values ofone and the same grey level. Stated otherwise, the generation of falsecontours due to the use of pairs is also decreased.

By way of experiment, if we take the pairs comprising on the one handthe grey level 130 and on the other hand one of the grey levels lyingbetween 0 and 255, the person skilled in the art can appreciate that in75% of cases the coding of the level 130 is performed on the twelvesubscans of low weight. Although the level 130 is coded according tosixteen different codes, the distribution of subscans remains groupedand homogeneous, thereby eliminating any false contour effect.

In 22% of possible cases, one of the two subscans of high weight is usedto code the value 130. However, the various codings exhibit homogeneousdistributions over the eleven subscans of low weight which minimize thefalse contour effect.

The cases corresponding to the most detrimental coding, that is to sayto the simultaneous use of the two subscans of high weight, correspondto 7% of the possible combinations. Another factor decreasing theawareness by the human eye of this coding defect stems from the factthat the pairs giving rise to such a defect correspond to a strongtransition which attenuates the false contour effect.

A preferred embodiment of the invention will now be described whilereferring to FIGS. 7 to 10. FIG. 7 represents an encoding device 200,according to the invention, serving to code the grey levels as a drivecode for the various subscans FSC and SSC. A plasma display panel cancomprise one or more devices of this type depending on the necessarycalculation time and the number of cells present on the said panel.

The encoding device 200 deploys first and second input buses, forexample eight-bit busses, for receiving the grey levels GL1 and GL2corresponding to two cells sharing the same second subscans SSC. Thegrey levels GL1 and GL2 may originate either from an image memorycontaining the entire image, or from a decoding device which performsthe decoding of a video signal and which translates it into a grey levelfor each cell. The encoding device 200 deploys three output buses whichsupply the words COM, Si and S2 which correspond respectively toignition or nonignition codes for the second subscans SSC, for the firstsubscans FSC associated with the first grey level GL1 and for the firstsubscans associated with the second grey level GL2.

The encoding device 200 comprises a difference circuit 201 whichreceives the two grey levels GL1 and GL2 to be encoded and supplies on afirst output the absolute value of the difference between GL1 and GL2.Moreover, on a second output of the said difference circuit 201, aninformation bit SeIC indicates which grey level GL1 or GL2 is to beregarded as greater than the other.

The difference circuit 201 is for example constructed as indicated inFIG. 8. First and second subtraction circuits 301 and 302 receive thegrey levels GL1 and GL2 on opposite inputs, so that the firstsubtraction circuit 301 supplies on a result output the differenceGL1−GL2 and so that the second subtraction circuit 302 supplies on aresult output the difference GL2−GL1. The second subtraction circuitfurthermore deploys an overflow output (also known as a carry output)which makes it possible to ascertain whether the result of thesubtraction is positive or negative and hence supplies the informationbit SelC. A multiplexer 303 receives the information bit SelC on aselection input and deploys first and second inputs connected to theresult outputs of the first and second subtraction circuits 301 and 302respectively. The multiplexer 303 selects the positive result as afunction of the information bit SelC so that the output of themultiplexer 303 corresponds to the output of the difference circuit 201.

The encoding device 200 furthermore comprises a first comparison circuit202 which compares the absolute value of the difference |GL1−GL2| withthe maximum difference value DMAX which is fixed by the subscans used.The first comparison circuit 202 supplies a first selection signal SelAwhich corresponds to the result of the first test 104. The personskilled in the art may note that it is not necessary to round up to 5 inorder to perform this comparison since the final result remainsequivalent before or after rounding.

A rounding circuit 203 receives the absolute value of the difference|GL1−GL2| so as to round it to 5. A first output supplies the roundeddifference D and a second output supplies a rounding control bus. Therounding control bus indicates the manner in which the values V1 and V2are to be modified. The rounding circuit 203 can be embodied with theaid of a lookup table, some of the output bits of which correspond tothe rounded difference D and some of the output bits of which correspondto a control code.

A calculation circuit 204 receives the grey levels GL1 and GL2 andsupplies the values V1 and V2 which will be used for the coding. Forthis purpose, the calculation circuit 204 receives the information bitSeIC so as to match the highest level GL1 or GL2 with the value V1 andthe lowest level GL1 with the value V2. The calculation circuit 204 alsoreceives the control bus originating from the rounding circuit 203 soas, if necessary, to carry out an addition or a subtraction of one uniton V1 and/or V2.

A first coding circuit 205 receives the value V1 and supplies a completecoding of this value on the entirety of the subscans FSC and SSC.However, a six-bit bus supports the word SPEMAX corresponding to thefirst subscans FSC and an eight-bit bus supports the word COMMAXcorresponding to the second subscans SSC. For the sake of simplicity andspeed of calculation, a lookup table which contains an optimum coding isused.

A second coding circuit 206 receives the value V2 and supplies a codingof this value on the second subscans SSC only. The output bus of thissecond coding circuit 206 supports the word COMMIN. This second codingcircuit can also be embodied with the aid of a lookup table. The personskilled in the art will note that only a limited number of differentvalues actually need to be coded and that it is therefore not necessaryto use a table deploying more than seven input bits.

A second comparison circuit 207 receives on this one hand the roundeddifference D and on the other hand the word SPEMAX. This secondcomparison circuit will compare the value associated with the wordSPEMAX with the rounded difference D so as to supply on a first output asecond selection signal SeIB which corresponds to the result of thesecond test 106. The second comparison also supplies on a second outputa six-bit word corresponding to the first subscans and having anassociated illumination which corresponds either to the roundeddifference D or to the illumination associated with SPEMAX from whichthe rounded difference D has been deducted.

An exemplary embodiment of the second comparison circuit 207 isrepresented in FIG. 9. The second comparison circuit 207 comprises adecoding circuit 401, a subtraction circuit 402, a multiplexer 403 and acoding circuit 404. The decoding circuit 401 is for example a lookuptable receiving the six bits of the word SPEMAX and supplying aneight-bit word corresponding to the value representative of theillumination associated with the word SPEMAX. The subtraction circuit402 deploys two inputs respectively receiving the rounded difference Dand the word exiting the decoding circuit 401, so that it supplies onits output the result of the difference corresponding to the value ofSPEMAX−D. The subtraction circuit 402 deploys an overflow output orcarry output which indicates whether the result of the subtraction ispositive or negative. The said overflow output thus supplies the secondselection signal SelB. The multiplexer 403 deploys two input busesrespectively receiving D and the result exiting the subtraction circuit402. A selection input of the multiplexer 403 receives the secondselection signal so that the output bus of the multiplexer 403 suppliesD if D is greater than the value of SPEMAX or the result exiting thesubtraction circuit 402 otherwise. The coding circuit is a lookup tablewhich deploys an input connected to the output of the multiplexer 403 soas to receive, either D, or SPEMAX−D, in order to supply a six-bit codecorresponding to the coding of the input value with the aid of the firstsubscans FSC.

The encoding device 200 comprises a selection circuit 208 which receiveson the one hand the various words COMMIN, COMMAX, SPEMAX and the wordexiting the second comparison circuit, and on the other hand the firstand second selection signals SelA and SelB. The said selection circuitsupplies, on first and second outputs, first and second words Si and Sjof six bits corresponding to the codings of the first subscans for thetwo grey levels GL1, GL2, and, on a third output, a third word COM ofeight bits corresponding to the common coding of the second subscans forthe two grey levels GL1 and GL2

An exemplary embodiment of the selection circuit 208 is shown in FIG.10. The circuit 208 comprises a decoder 501 and three multiplexers 502to 503. The decoder circuit 401 receives the first and second selectionsignals SelA and SelB and supplies the controls necessary for the threemultiplexers 502 to 504 in order to perform the branchings defined inthe fourth to sixth steps 105, 107 and 108.

The multiplexer 502 deploys two inputs which receive the words COMMINand COMMAX and an output which supplies the word COM. The word COMcorresponds, either to COMMIN when the first selection signal SelAindicates that D is greater than DMAX or when the second selectionsignal SelB indicates that the value of SPEMAX is less than D, orcorresponds to COMMAX when the first and second signals indicate that Dis not greater than DMAX and that the value of SPEMAX is not less thanD.

The multiplexer 503 deploys two inputs and one output. One of the saidinputs receives the six-bit word originating from the second comparisoncircuit 207, the other of the said inputs receives a six-bit wordcorresponding to the zero value encoded for the first subscans, and theoutput supplies the word Sj. The word Sj corresponds, either to the zerovalue coded on six bits when the first selection signal SelA indicatesthat D is greater than DMAX or when the second selection signal SelBindicates that the value of SPEMAX is less than D, or corresponds to theword exiting the second comparison circuit 207 COMMAX when the first andsecond signals SelA and SelB indicate that D is not greater than DMAXand that the value of SPEMAX is not less than D.

The multiplexer 504 deploys first to third inputs and an output. Thefirst input receives a word corresponding to DMAX, that is to saycorresponding to the maximum illumination possible with the aid of thefirst subscans FSC. The second input receives the word SPEMAX. The thirdinput receives the word exiting the second comparison circuit 207. Theoutput supplies the word Si which corresponds, either to the wordcorresponding to DMAX when the first signal SelA indicates that D isgreater. than DMAX, or SPEMAX when the first and second signals SelA andSelB indicate that D is not greater than DMAX and that SPEMAX is notless than D, or the word exiting the second comparison circuit when thesignals SelA and SelB indicate that D is not greater than DMAX and thatSPEMAX is less than D.

The encoding device 200 comprises an output circuit 209 which receivesthe words Si and Sj so as to match them either respectively to the wordsS1 and S2, or respectively to the words S2 and S1 as a function of theinformation bit SelC.

The encoding device 200 is then incorporated into a display panel 600 soas to allow image display 601, as represented in FIG. 11.

Such an encoding device 200 can be embodied according to differentvariants. By way of example, if the person skilled in the art deems thecalculation time to be too small, it is for example possible to adopt apipeline type structure. Accordingly, it is for example possible to addextra storage registers 210 as indicated in FIG. 7 so as to perform thecalculation in two stages, thereby allowing an overall reduction in thecalculation time for an image.

Numerous other alternatives are available to the person skilled in theart. In the preferred example, lookup tables are used to perform thecodings and decodings for reasons of simplicity of implementation andhence of reliability. It goes without saying that these lookup tablescan be replaced by calculation circuits, in particular if it is chosento implement such a device with the aid of microcontroller typecircuits.

More generally, the person skilled in the art may also elect to carryout the method of the invention solely with the aid of programmedcircuits essentially comprising a processor and a memory. The devicethus embodied will deploy a totally different structure from the devicerepresented.

Also, in the present description of the invention, reference is made toa coding using six first subscans whose respective weights are 5, 10,20, 30, 40 and 45 and eight second subscans whose respective weights are1, 2, 4, 7, 13, 17, 25 and 36. This coding has been chosen for thepresent description since it enables good results to be obtained. Noreference has been made to other types of coding during the descriptionfor reasons of clarity, but it is obvious that other types of coding maybe used with the same method provided that certain numerical values arechanged.

By way of complementary example, it is for example possible to use acoding for sixteen subscans comprising four first subscans whoserespective weights are 5, 10, 20 and 35 and ten second subscans whoserespective weights are 1, 2, 4, 6, 9, 12, 15, 19, 23, 27, 31 and 36,taking care to modify accordingly the various quantities (value of DMAX,number of coding bits) which have been used in the description.

What is claimed is:
 1. Method of displaying a video image on a plasmadisplay panel for a duration of display, the said panel comprising aplurality of cells arranged in rows and columns, each cell being lit fora duration lying between zero and a maximum display time correspondingto the maximum brightness of a cell for a given brightness setting, thetotal time of illumination of a cell being divided into severalillumination periods corresponding to various subscans among which aredistinguished first subscans specific to an addressing of each cell andsecond subscans common to two cells arranged on neighbouring rows, suchthat, for a pair of cells sharing the same second subscans, the greylevels GL1 and GL2 of the said cells are decomposed into a common valueCV and into a specific value SV1 and SV2 with the aid of the followingequation: GL1=CV+SV1 and GL2=CV+SV2, with GL1 greater than or equal toGL2, the common value CV corresponding to the sum of the weights of thesecond subscans, the specific values SV1 and SV2 corresponding to thesum of the weights of the first subscans associated respectively withthe grey levels GL1 and GL2, wherein the following steps are carriedout: E1: coding of the highest grey level GL1 over the entirety of thesubscans while favouring the subscans whose illumination time is thesmallest; E2: extraction of a specific coding value corresponding to thesum of the first subscans used during the coding of step E1; E3: if thespecific coding value extracted in step E2 is less than the differenceGL1−GL2, then the values SV1 and CV correspond to the coding carried outin step E1, and the resulting value SV2 equal to GL2−CV is coded on thesecond subscans while favouring the subscans of low weight.
 2. Methodaccording to claim 1, wherein the following step is carried out: E4: ifthe specific coding value extracted in step E2 is less than thedifference GL−GL2, then the common value CV is equal to the lowest greylevel GL2 and then a new value SV1=GL1−GL2 is calculated.
 3. Methodaccording to claim 1, wherein, if the maximum value encodable with theaid of the first subscans is less than the difference GL1−GL2, then thecommon value CV is equal to the lowest grey level GL2, and the specificvalue SV1 is equal to the maximum value encodable on the first subscans.4. Method according to claim 1, wherein prior to any coding operation,the value 1 may possibly be added to and/or subtracted from one or bothgrey levels GL1 et GL2 so that the difference GL1−GL2 is a multiple offive.
 5. Method according to claim 1, wherein the display durationsassociated with the first subscans correspond to the product of anelementary duration times respectively the factors: 5, 10, 20, 30, 40,45, and in that the display durations associated with the secondsubscans correspond to the product of the elementary duration timesrespectively the factors: 1, 2, 4, 7, 13, 17, 25,
 36. 6. Plasma displaypanel comprising a plurality of cells arranged in rows and columns, eachcell being lit for a duration lying between zero and a maximum displaytime corresponding to the maximum brightness of a cell for a givenbrightness setting, the total time of illumination of a cell beingdivided into several illumination periods corresponding to varioussubscans among which are distinguished first subscans specific to anaddressing of each cell and second subscans common to two cells arrangedon neighbouring rows, such that, for a pair of cells sharing the samesecond subscans, the grey levels GL1 and GL2 of the said cells aredecomposed into a common value CV and into a specific value SV1 and SV2with the aid of the following equation: GL1=CV+SV1 and GL2=CV+SV2, withGL1 greater than or equal to GL2, the common value CV corresponding tothe sum of the weights of the second subscans, the specific values SV1and SV2 corresponding to the sum of the weights of the first subscansassociated respectively with the grey levels GL1 and GL2, wherein itcomprises a grey level encoding device comprising: a first codingcircuit for coding the highest grey level GL1 over the entirety of thesubscans while favouring the subscans whose illumination time is thelowest; a means for extracting a specific coding value exiting the firstcoding circuit; a selection and calculation circuit for carrying out thecoding of the lowest grey level by using the common value CV exiting thefirst coding circuit if the specific coding value extracted from thefirst coding circuit is greater than the difference GL1−GL2.
 7. Panelaccording to claim 6, wherein it comprises a second coding circuit forcoding the common value CV as being equal to the lowest grey level GL2,and in that, if the specific coding value extracted from the firstcoding circuit is less than the difference GL1−GL2, the selection andcalculation circuit determines a new value SV1=GL1−GL2.
 8. Panelaccording to claim 6, wherein it comprises a comparison circuit forcomparing the difference GL1−GL2 with a maximum value encodable on thefirst subscans, and in that the selection and calculation circuitdetermines that SV1 is equal to the maximum value encodable on the firstsubscans and that the common value is equal to the lowest grey level. 9.Panel according to claim 6, wherein it comprises a rounding circuitplaced upstream of the first and second coding circuits, the saidrounding circuit possibly adding the value 1 to or subtracting the value1 from one or both grey levels GL1 and GL2 so that the differenceGL1−GL2 is a multiple of five.